What cards can I upgrade the Geforce GO 8400M GS to?

The 8600 M is the only upgrade option. You can wait, but if history holds, there won't be a more advanced option - what you see at the introduction of the system, is what you get permanently, and since the 1720 is due for replacement by the 1725, which looks like it'll have the same video options, that's likely to remain the terminal upgrade for this model.

I'd guess about $250; you'll have to call Dell for an exact price.

Dell Spare Parts 1-800-449-3355 ext 7269937